Alkyl hydroxyalkyl cellulose ethers and processes for preparing them are generally known in the art. The alkyl hydroxyalkyl cellulose ethers find use in a large variety of technology areas including the building, pharmaceutical and food industry and in a wide range of applications, for example in pharmaceutical capsules and tablets, or as suspending agents in vinyl chloride polymerization. Depending on the type of end-use, the alkyl hydroxyalkyl cellulose ethers vary in molecular weight, type or degree of alkyl and hydroxyalkyl groups present in the cellulose ether or combinations thereof.
A commonly known process for preparing alkyl hydroxyalkyl cellulose ethers includes the steps of alkalizing cellulose and etherification of the alkali cellulose with an alkylene oxide and an alkyl halogenide. The etherification with an alkylene oxide requires the presence of a catalytic, that means sub-stoichiometric, amount of a base, whereas the etherification with an alkyl halogenide consumes a stoichiometric amount of a base.
GB-A-1,003,662 discloses a process for preparing alkyl hydroxyalkyl cellulose ethers wherein cellulose is pre-treated with an alkali halide in the presence of alkali and water until a D.S. value of 0.05 to 0.5 is attained, the amount of alkali contained in the pre-treated cellulose is reduced to less than 10 percent, based on the pre-treated cellulose and the pretreated cellulose is contacted with a gaseous alkylene oxide in the presence of the reduced amount of alkali until an M.S. value of more than 1.0 is attained. Pre-treatment can be conducted in the liquid phase, but that it is more conveniently conducted in the gaseous phase. GB-A-1,003,662 teaches that the reaction of the pre-treated cellulose ether with the alkylene oxide can be conducted in a methanol-acetone mixture, but for achieving an M.S. of more than 1 and a good solubility of the resulting cellulose ether, the pre-treated cellulose ether should be reacted with gaseous alkylene oxide. Unfortunately, the produced cellulose ethers are hot-water-soluble or thermoplastic and thus are not very useful for many important applications. Furthermore, as taught in GB-A-1, 003,662, the pre-treated cellulose should be contacted with a gaseous alkylene oxide for etherification. If the etherification with propylene oxide is carried out in liquid phase in the presence of a mixture of methanol and acetone, as disclosed in the comparative process in Example 1, propylene oxide will react with methanol to produce, for example, methoxypropanol or 1,2-dimethoxypropan as undesirable by-products. Moreover, any further purification, which is necessary for many applications, of the thermoplastic or hot-water-soluble cellulose ether produced in liquid phase by washing with hot water presents major difficulties. Moreover, the produced alkyl hydroxyalkyl cellulose ethers with a D.S. value of only from 0.05 to 0.5 are not useful for many applications in the building and other industries.
German Offenlegungsschrift DE-A-33 16 124 (corresponding to U.S. Pat. No. 4,550,161) teaches that the process disclosed in GB-A-1,003,662 has substantial disadvantages. It teaches that the absence of liquid dispersants leads to inhomogeneously etherified products and that the cellulose which has been pre-treated with an alkali halide has to be purified prior to the reaction with an alkylene oxide because of the high content of residual alkali. To overcome these disadvantages, German Offenlegungsschrift DE-A-33 16 124 discloses a process with the following steps: (a) alkalizing the cellulose ether, (b) etherifying the alkali cellulose with ethylene oxide or propylene oxide in the presence of a sub-stoichiometric quantity of a base, (c) increasing the quantity of the base and (d) etherifying the cellulose ether with methyl chloride. In all steps water is present. In at least one step dimethoxyethane, an alkanol, an alkane diol and/or an alkoxyalkanol is used as a dispersing auxiliary.
In U.S. Pat. No. 4,650,863 a similar process is disclosed except that the dispersing auxiliary is dimethyl ether, which is used either alone or in mixture with dimethoxyethane, an alkanol, an alkane diol and/or an alkoxyalkanol.
Unfortunately, in the processes taught in U.S. Pat. Nos. 4,550,161 and 4,650,863 the starting cellulose pulp is insufficiently activated by the low quantity of the used caustic leaving too many crystalline areas of the cellulose fibers intact.
In other known, commonly used processes the etherification of the alkali cellulose with an alkylene oxide and an alkyl halogenide are conducted simultaneously.
According to the procedure described in U.S. Pat. Nos. 4,550,161 and 4,650,863 the free hydroxy groups of the anhydroglucose units in the cellulose react with an alkylene oxide, which means that they are etherified with hydroxyalkyl groups. The hydroxyalkyl substituents again contain a free hydroxy group. Therefore, there are different kinds of free hydroxy groups available after hydroxyalkylation. There are free hydroxy groups in the anhydroglucose and in the hydroxyalkyl groups as well. Depending on the employed amount of alkyl halogenide for the alkylation, both kinds of the free hydroxy groups are etherified with alkyl groups. In case of the etherification of the hydroxyalkyl groups the common term is capping. It is difficult or very circumstantial to control which hydroxy groups are capped with alkyl groups.
In view of the deficiencies of the prior art processes, it would still be desirable to provide a new process for producing alkyl hydroxyalkyl cellulose ethers. For enriching the art, it would particularly be desirable to provide new alkyl hydroxyalkyl cellulose ethers.